If you look at the north and south sides where there are large swathes of green residential neighborhoods, you can see they are regularly divided by blue commer

If you look at the north and south sides where there are large swathes of green residential neighborhoods, you can see they are regularly divided by blue commercial corridors along the cardinal directions. Chicago's grid design was adapted from the original Northwest Ordinance, which divided the land into square mile sections. Those blue strips are one mile apart, and in denser neighborhoods an additional blue strip exists midway at the half mile mark. This organizing principle is what made Chicago a city of neighborhoods, since each section or quarter section is bounded by commercial thoroughfares on each side, and the neighborhood turns inward. The section line and half section line thoroughfares are wider roads which permit more traffic volume. I think it's a good compromise between facilitating traffic flow and also isolating it from the contained neighborhoods where the bulk of people live. As far as freezing the zoning in place, that's not really true as downtown areas were once like the neighborhoods and were subsequently upzoned as density and desirability increased. The original zoning plan is intact in the neighborhoods because there just hasn't been a desire or need to upzone them (yet). Chicago is known as a relatively pro-development city which is flexible on things other cities are inflexible on.